HTLV-1 was the first defined human pathogenic retrovirus. Seroepidemiologic and molecular genetic data firmly establish an association between HTLV-1 infection and a chronic, slowly progressive neurologic condition previously termed tropical spastic paraparesis (TSP) that often emanated from clusters in tropical areas, including the Caribbean Basin, India, and Africa. A similar condition described in Japan is referred to as HTLV-1-associated myelopathy (HAM). It is now recognized that HAM and TSP (HAM/TSP) are identical clinical conditions. The clinical course in HAM/TSP is predominantly that of a slowly progressive spastic paraparesis associated with bladder and bowel dysfunction. Importantly, this disease shares many clinical similarities with the primary progressive form of multiple sclerosis (MS). Indeed, many HAM/TSP patients have been diagnosed as MS up until the time that HTLV-I has been dected in sera or cerebrospinal fluid (CSF). The histopathological changes in HAM/TSP, again similar to MS, include a considerable mononuclear cell inflammation with lymphocytic perivascular cuffing, capillary proliferation, demyelination, and a reactive astrocytosis. Demyelination and axonal damage affects the spinal cord most severely, predominantly in the lateral columns. However, the entire neuroaxis may be involved including optic nerves, cerebrum, cerebellum, brainstem and peripheral nerves. Regulation of HTLV-1 production is mediated by non-structural gene products, particularly through the transcriptional activator known as HTLV-1 Tax. This transactivating protein induces the expression of host cellular genes, especially those that play crucial roles in cell proliferation and differentiation, such as interleukin-2 and its receptor. This in turn likely contributes to a state of ongoing lymphocyte activation seen in this disease. HTLV-1 has minimal direct cytopathic effect and the observed tissue destruction in HAM/TSP is thought to be a consequence of immune mediated pathological events. In addition, clinically manifest disease attributable to HTLV-1 such as HAM/TSP is seen in only a small percentage of infected individuals. This suggests the contribution of several susceptibility factors including genetic and immunological makeup of infected individuals in addition to viral characteristics. Antibodies to HTLV-1 can be detected in sera and CSF of infected patients by using enzyme linked immunosorbent assays (ELISA). Radioimmunoprecipitation assay (RIPA) or Western Blotting techniques are used to confirm positive HTLV-1 results. The World Health Organization accepts for diagnosis a positive HTLV-1 ELISA followed by a Western Blot with antibody banding with p19 or p24 encoded protein in combination with banding for rgp21 or rgp46env protein. Individuals with a positive HTLV-I ELISA but a Western Blot that only partially fulfills the above criteria are considered to be sero-indeterminate. Whether this incomplete banding pattern in sero-indeterminate individuals is due to defective retrovirus or due to low virus load has been a subject of our investigation. We are investigating novel techniques for the detection of antibodies to different HTLV-I proteins in patients infected with HTLV-I and seroindeterminates. We have demonstrated HTLV-I specific antibody banding patterns that can differentiate between HTLV-I infected individuals with and without neurological disease. A major emphasis within the Viral Immunology Section is to examine clinical, virological and immunological parameters in individuals with HAM/TSP, asymptomatic sero-positive individuals, family members at risk for acquiring HTLV-1, and individuals with HTLV-1 indeterminate serology. Specific focus is on the effect of viral load, presumed route of infection and genetic makeup on immunological function. Whole blood, lymphocytes, and plasma/serum obtained from individuals with HAM/TSP, asymptomatic sero-positive individuals, sero-indeterminate individuals, as well as, healthy controls. We will assess virological and immunological parameters using these samples. In HTLV-1 seropositive individuals these studies will be extended to cerebrospinal fluid. Basic laboratory investigations have demonstrated the importance of the cytokine IL-15 in the life and death of lymphocytes and for its role in autoimmune disorders such as HAM/TSP. IL-15 is pivotally involved in the survival of CD8+ memory T-cells including self-directed cells and we have shown that in HAM/TSP, IL-15 is essential for the survival of HLA class I restricted virus antigen-specific effector and memory CD8+ T-cells. These CD8+ antigen-specific CTL are thought to play a major role in the immunopathogenesis of HAM/TSP since they have been localized in brain and spinal cord sections of patients. As IL-15 is a pro-inflammatory cytokine that stimulates the production of TNF&#945;, IL-1&#946;and other inflammatory cytokines, the release of IL-15 induced by HTLV-I tax in patients with HAM/TSP may underlie the pathogenesis of this autoimmune disease. The mode of action of IL-15 and its receptor subunits IL-15 and IL-15Ralpha are coordinately stimulated and expressed following HTLV-I tax stimulation. We have shown that this trans-stimulation can be virtually totally inhibited for NK and CD8+ T-cells by the addition of a humanized monoclonal antibody, Hu MiK-&#946;1, that blocks IL-15 binding to the IL-2/IL15R&#946;receptor subunit expressed on these cells. Collaborative research by the Viral Immunology Section, NINDS and the Metabolism Branch, NCI have initiated a phase I clinical trial for the treatment of HAM/TSP using Hu MiK-beta-1 that blocks the action of IL-15. We have assessed the effects of intravenously administered Hu-MiK-beta-1 on the cellular immune response in patients with HAM/TSP, with particular focus on virus-specific memory CD8+ T cells. Secondary outcomes to be measured are clinical responses, including toxicity, and the effect on CD4+CD25+ T regulatory cells. As IL-15 over expression has been demonstrated in patients in a wide variety of autoimmune disorders including, rheumatoid arthritis, inflammatory bowel disease, multiple sclerosis and psoriasis, the successful application of Hu MiK-beta-1 in HAM/TSP has significant and broad therapeutic implications. The HTLV-I induced activation of nuclear factor kappa B (NFkB) is considered a key event in the leukemogenesis leading to ATLL, but less has been demonstrated regarding the role of NF&#954;B activation in the pathogenesis of HAM/TSP. Herein we show evidence of both canonical and non-canonical NF&#954;B activation in short-term cultures of PBMC from subjects with HAM/TSP, in the absence of exogenous stimulation. We show that the NFkB activation in HAM/TSP PBMC can be reversed by small molecule inhibitors. Inhibition of NFkB activation in HAM/TSP PBMC resulted in the inhibition of STAT5 activation and spontaneous lymphoproliferation, two ex vivo correlates of the immune activation associated with HAM/TSP. These results indicate that NFkB activation plays a critical upstream role in the immune activation associated with HAM/TSP, and identify the NFkB pathway as a potential target for immune modulation in HAM/TSP. A number of laboratory studies have demonstrated that virus infection can activate a variety of CNS-specific cells including microglia and astrocytes. The VIS has investigated the effects of both a human retrovirus (HTLV-I) and herpesvirus (HHV-6) in these cell populations, in vitro. We are translating these observations to the patient by imaging studies that can detect activated microglia/macrophage and reactive astrocytes. In collaboration with the Molecular Imaging Branch, NIMH, we have begun to investigate the expression of the peripheral benzodiazepine receptor (PBR) in the CNS that has been show be a marker of activated microglia/macrophage.